The first couple, said to have known links to jihadists, were detained at a service station in the south of France. Another couple were picked up in the Loiret region to the south of Paris late on Wednesday.

Both couples were thought to come from the town of Montargis. The car's owner was questioned and later released, but anti-terrorist police are searching for his 19-year-old daughter who is described as radicalised.

In a lengthy speech on Thursday, President Hollande said that it was incumbent on all of France - Muslims and non-Muslims - to tackle radical Islam.

He launched a defence of France's strict secular separation of the state from religion. "Nothing in the idea of secularism opposes the practice of Islam in France, provided it respects the law."

Attack on Sarkozy

The speech was being closely watched by political commentators, who said he had given his biggest signal so far of planning to stand for a second term in next year's presidential election.

Image copyrightAFPImage caption
Nicolas Sarkozy will formally become a candidate for the Republicans' presidential nomination on Friday

Although he is widely expected to run, Mr Hollande's approval ratings have fallen so low that opinion polls suggest as many as nine out of 10 French voters do not want him as a candidate.

Commentators noted that the speech included a series of attacks on the centre-right Republicans, in particular on ex-President Nicolas Sarkozy, a front-runner in the race to be Republican candidate.

Without naming him, Mr Hollande criticised a remark made by Mr Sarkozy after the murder of a priest in Rouen and of 86 people on the seafront at Nice in July.

Mr Sarkozy called at the time for strong measures, complaining that "legal niceties, cautious measures and excuses for half-hearted measures are intolerable".

But in his speech, President Hollande argued that constitutional principles such as freedom of expression, movement and worship were not "legal niceties". And, in a direct reference to Mr Sarkozy's own legal troubles, he asked: "Is being presumed innocent (a legal nicety)? It's pretty useful when you're defending yourself."